This invention is directed to relays and, more particularly, to control circuits for controlling the closure of relay contacts when AC is applied across the contacts.
In many electronic control systems AC power is applied to a load through relays. In many instances relatively low voltage control systems are utilized to sense and manipulate data and, in accordance with that data, generate control signals. The control signals, in turn, control the closure of relay contacts so that AC power is applied to the desired loads at the desired point in time. The loads may be in the form of relatively large banks of lamps (lampbanks), adapted to display information in numeric or alphanumeric form, visible at relatively large distances such as by personnel located in an N/C machine tool area. Alternatively, the load may comprise electric motors or other electric devices requiring relatively high voltage power. The AC power may be single phase or multi-phase (e.g., three phase) power depending upon the nature and requirements of the particular load.
One of the major disadvantages of relays controlling AC is the arcing that occurs when the relay contacts are closed at some point other than the zero crossing point in the AC waveform. More specifically, if relay contacts are closed when a relatively high voltage is applied across the contacts, arcing occurs. Arcing causes deterioration of relay contacts and, thus, reduces relay life. In order to overcome this disadvantage, in recent years, relatively high voltage semi-conductor devices (e.g., silicon controlled rectifiers, triacs, etc.) have replaced relays in many electrical systems. However, these devices have voltage and power limits that prevent their use in many other systems. Moreover, in many instances, particularly when high power is involved, these devices are more expensive than desirable. Thus, while relays have certain undesirable features, they also have certain desirable features, particularly in the area of cost and power transfer capability. In some situations they are the only device available for use.
It will be appreciated from the foregoing brief discussion that it would be desirable to provide a circuit for controlling relay contact switching such that the instantaneous AC voltage existing across the contacts is at a minimum level (preferably zero) when the contacts close. Obviously, switching at a minimum voltage level will result in lower arcing and, thus, longer contact life.
Therefore, it is an object of this invention to provide an AC relay control circuit.
It is a further object of this invention to provide an AC relay control circuit adapted to sense the impending occurrence of the zero crossing point of an AC voltage, and control the closure of relay contacts at the zero crossing point.
It is another object of this invention to provide an inexpensive, yet reliable, zero crossing AC relay control circuit.
It is a still further object of this invention to provide an AC relay control circuit adapted to prolong the life of the contacts of relays.